user-manual: Import YP Docs templates for usermanual improvements

Import the necessary pieces to be able to build the BitBake User
Manual using make in a similar way that the Yocto Documentation is
built.  The Makefile has been edited to remove Yocto Project specific
content and adapt for bitbake's needs.

Signed-off-by: Bill Traynor <wmat@alphatroop.com>
user-manual: Separate out chapters into individual files

Splitting the manual into a chapter per file makes the content
a little more managable.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
gitignore: Update for recent docs changes

Added a few more files to gitignore file to allow document
generation testing.

Signed-off-by: Bill Traynor <wmat@alphatroop.com>
documentation: Add README based off the YP docs one

Reusing the README file from the Yocto Project Documentation
directory but removed the YP specific content and changed to be
specific to BitBake.

Signed-off-by: Bill Traynor <wmat@alphatroop.com>